In the development of aerospace hardware it is often necessary to interconnect two or more structural panels or elements while permitting limited relative movement caused by thermal expansion or contraction of the parts during extreme temperature cycling as encountered in space flight. Thermal expansion or contraction of any arbitrary point in a homogeneous material at a uniform temperature relative to any reference point is directly proportional to the radial distance between the arbitrary point and the reference point. Restraint of the thermal expansion or contraction by a second homogeneous material surrounding the first homogeneous material will produce thermal stresses in both materials. This thermal stress condition will exist when the extension of line elements of all mating or contacting surfaces between the two materials do not pass through a common reference point and when the two materials have different coefficients of thermal expansion (when the temperatures of both materials is changed from some initial temperature) and when the materials each possess a modulus of elasticity.
If the coefficient of thermal expansion of the first homogeneous material is greater than that of the surrounding material, thermal stress will be generated when the materials are heated. Also, if the coefficient of thermal expansion of the first homogenous material is less than the surrounding material, thermal stresses will be generated when the materials are cooled.
With a surrounding material initially contacting the homogenous material by mating surfaces (whose line element extensions do not pass through a common reference point when extended), separation of the materials will occur as the temperature is changed from the initial temperature provided: (1) when the coefficient of thermal expansion of the homogenous material is greater than that of the surrounding material, the materials will separate when cooled; (2) when the coefficient of thermal expansion of the homogeneous material is less than the surrounding material, the materials will separate when heated.
Maintaining contact between materials of different coefficient of thermal expansion as the temperature fluctuates may be as significant to the successful operation of structural devices utilizing these laws of physics as the avoidance of restraint of expansion.
To prevent either thermal stresses in, or separation of, the materials it is essential that the extension of line elements of all mating surfaces of the joined materials pass through a common reference point. The mating surfaces may be any combination of planes, cones and pyramids. Moreover, the line elements, when extended, of at least two such of all other mating surfaces must pass through the reference point and when the extended line elements of all other surfaces do not pass through the reference point these surfaces are required to be separated to avoid thermal stress when the thermal expansion is such to cause these mating surfaces to interfere with one another.
U.S. Pat. No. 4,229,018 issued to Bickerstaff et al on Nov. 10, 1981, applies some of the principles of the present invention although the structure and function of the respective devices are significantly different. Bickerstaff et al disclose a roll with tires clamped to it by frusto-conical rings of the same material composition as the roll shaft but different from the tires. The entire roll assembly rotates as it and others of like construction are pulled through and along the floor of an oven while transporting a sheet of glass resting thereon. The matching frusto-conical contact between the retaining rings and tires are designed so that differential thermal expansion between these parts causes sliding movement only of these frusto-conical surfaces relative to each other.
It is therefore an object of the present invention to provide a fastener that extends through two or more structural element parts that are to be connected in such a manner as to avoid thermal stresses in the connected area of the connected parts during thermal cycling.
It is a further object of the present invention to provide a fastener for alleviating thermal stresses in connected structural parts wherein the fastener and structural parts have substantially different coefficients of thermal expansion.
A further object of this invention is a fastener extending through two or more structural parts connected thereby wherein fastener/structural part contact is along mating planes.
A further object of the present invention is to to provide a connector element for two or more structural parts that maintains contact between the parts connected and the mating surfaces of the fastener during thermal cycling.
An additional object of the present invention is a fastener for extending through and connecting two or more structural elements wherein the fastener/structural element contact is along mating surfaces and an extension of the line elements of the mating surfaces pass through a common point.